Nigel.. As you know, I'm retired so it would be as well to obtain current info and hopefully one of my ex-colleagues will comment. However......
<<In short, does not seem you "expect" aircraft to be able to "divert"?>>
Not wholly sure what you mean. Go-arounds are fairly commonplace and ATC treats them all as "expecting" to be repositioned for landing at Heathrow unless the pilot says otherwise. A diversion to another airfield can be accomplished by ATC with the minimum effort from anywhere in the circuit. But... if you mean "divert" to the other runway then you should be aware that this may not be a possibility.
Procedures may well have changed but I always assumed that it was inappropriate to issue a SID to a go-around (remember we're forever being reminded about pilot "workload"!). I always worked on the basis that a diversion of a late go-around just got radar vectors but someone who stated that he might divert after a go-around (LVPs) got a SID well in advance.